The Chemical Properties of the Halogen Elements and the Hydrogen Halides

Full Notes

Reactivity of Halogens as Oxidising Agents

Halogens can act as oxidising agents by gaining electrons to form halide ions (X⁻).

Trend:
Oxidising power decreases down the group: Cl₂ > Br₂ > I₂.

Why:

• Down the group, atomic radius increases.

• More electron shielding.
• Weaker attraction for incoming electrons.

Reactions of Halogens with Hydrogen

All halogens react with hydrogen (H₂) to form hydrogen halides (HX).

General equation: H₂ + X₂ → 2HX

Relative reactivity:

• Cl₂ reacts explosively in sunlight.
• Br₂ reacts less violently.
• I₂ forms HI slowly on heating.

Trend: Reactivity decreases down the group; reaction becomes less exothermic.

Thermal Stability of Hydrogen Halides

Hydrogen halides (HF, HCl, HBr, HI) are covalent gases that can be thermally decomposed at high temperatures.

Trend in thermal stability:
HF is most stable, Stability decreases down the group: HF > HCl > HBr > HI

Why:

• H–X bond strength decreases down the group.

• Bond length increases as atoms get larger.
• Shared electron pair is further from nuclei → weaker attraction.

Relative bond strengths:

• H–F: strongest (shortest bond).
• H–I: weakest (longest bond).

Result:

• HI decomposes most easily: 2HI → H₂ + I₂.
• HF remains stable even at high temperatures.

Summary

• Halogens are oxidising agents; oxidising power decreases Cl₂ → I₂.
• Halogens react with hydrogen to form HX; reactivity decreases down the group.
• Thermal stability of hydrogen halides decreases from HF to HI because bond length increases and bond strength decreases.